1. Field of the Invention
The present invention relates to a dental cement composition. Precisely, the invention relates to a dental cement composition, which, after being cured, is stable and bonds well to hard tissues such as tooth enamel and dentin and to prostheses such as inlays, onlays, crowns, cores, posts and bridges that are formed of metals, porcelains, ceramics and composite resins, and which is therefore useful in restoring decayed or injured teeth and in bonding prostheses.
2. Discussion of the Background
Amalgam, glass ionomer cement and composite resin are widely used for restoring decayed or injured teeth. On the other hand, zinc phosphate cement, glass ionomer cement and composite resin cement for bonding prostheses such as crowns, inlays and bridges to decayed or injured teeth. In that manner, various materials are used for restoring decayed or injured teeth, and one of them is glass ionomer cement.
The basic components of glass ionomer cement are a polyalkenoic acid, water and an ion-leachable glass filler. Attempts to improve the properties of glass ionomer cement, by adding other components, for example, are described in Japanese Patent Laid-Open Nos. 164807/1990, 6358/1990, 255033/1993, 26925/1996, and International Patent Publication Nos. 505868/1998, 506127/1998. The improved glass ionomer cements described therein are generally referred to as resin-modified glass ionomer cements, and which become popular in the art.
The basic principle in the restoration of defective teeth is to fill an aesthetic material having a certain strength in the cavity of a defective tooth or in the space between defective teeth and a prosthesis to ensure the permanence of the restored teeth, not to merely improve the function and shape of the restored teeth temporarily. Another purpose is to prevent the recurrence of caries.
In general, it is known that glass ionomer cement releases fluorine, which is effective for making teeth acid resistant from the viewpoint of caries prevention, but the fluorine release is only an auxiliary function of the glass ionomer cement. Specifically, if the cause of secondary caries is not eliminated, fluorine release, if any, to reinforce teeth could not be the essential solution to the problem of caries. To solve the problem of caries, it is first necessary to seal the interface between a restored tooth and a prosthesis with cement. This is to prevent caries-causing bacteria from invading the aforesaid interface and to prevent food residues that may provide the nutrients for the bacteria from entering, to thereby prevent secondary caries from recurring.
The function of fluorine may be taken into consideration merely as an auxiliary preventive means for bonding failure in the restored tooth. As reported in Quintessence, Vol. 16, No. 4 (1997), pp. 69–72, secondary caries accounts for about 50% of the reason for re-restoration of teeth once restored with glass ionomer cement, and is significant. This is because of mistaking the means for the end in that fluorine only is expected for caries prevention.
The oral environment to which tooth restorations are exposed is described. The oral cavity is always wetted with saliva circulating therein, and it is well known that, when plaque adheres to teeth, the area around the tooth becomes acidic owing to the acid produced by the bacteria in the plaque. Some food is acidic by itself. The oral temperature is generally equal to body temperature and is around 37° C., but this varies from around 0° C. to about 60° C. depending on the food taken. This means that teeth are exposed to heat shock. In addition, teeth receive some mechanical stress while they meet or while food is chewed. To that effect, tooth restorations are always in such extremely severe conditions. In the oral cavity that is generally in such a wet and acidic condition, glass ionomer cement will disintegrate, and the disintegration of the cement itself is a problem to be discussed before the bonding power of the cement to teeth and prostheses to seal decayed teeth are discussed.
Accordingly, it is a matter of first importance to prevent the cement disintegration in that condition. In addition, increasing the bonding power of the cement in the bonding interface will achieve tooth restorations of a higher level. However, conventional resin-modified glass ionomer cement do not fully solve the problems.